Systems for measuring sizes of particles by detecting scattered light from the particles as they pass through a beam of light are known in the prior art. Among the constant challenges are attempting to improve the resolution of the particle signals that are detected and reducing the amount of the background noise that is generated. The background that is generated can include several types of noise including molecular noise, shot noise, light fluctuation noise, photo-amp noise and turbulence noise. The type of noise that most influences the effectiveness of particle sensors is light fluctuation noise, especially in high power intracavity laser sensors. Fluctuations in the laser beam travel at the speed of light, and compared to the speed of the photo-amp, it is effectively fluctuating at the same time all along the beam. Air molecules in the view volume scatter the light fluctuation noise. It is desirable to eliminate this type of noise from particle sensing devices.
Advances in the development of optical particle sensors are continuously being sought after to increase the sensitivity of particle sensors especially at higher flow rates. It is desirable to achieve a higher flow rate without increasing the velocity of the sample stream. The methods described in U.S. Pat. Nos. 5,011,286 and 5,084,629 to Petralli, splits the flow into separate ports to reduce the velocity and background light level per detector. High particle velocity on the detector has two adverse effects, it creates flow turbulence (when the Reynolds number exceeds 2000) and it reduces the output signal because of photo-amp frequency response limitations. One way to maintain low velocity would be to use a larger nozzle to cover a large view volume in the sensor, such as the type described in U.S. Pat. No. 4,746,215 to Gross. However, for example, if the flow rate of a 0.1 cfm sensor was increased to 1.0 cfm, then the view volume and detector would need to be ten times larger. Because of the increased collection of background light noise and photo detector noise, the resulting larger detector would have a signal including ten times as much noise. A practical method to detect the larger view volume light scatter is to use arrays of small detectors. To reduce background light noise even further requires noise cancellation. Thus, the method of signal processing and noise cancellation is crucial to the performance of a sensor.
Various prior art approaches utilize detector arrays to increase the ratio of scattered light from a particle to the background Rayleigh light. These methods reduce background noise by using smaller detectors; therefore, less background is sensed per detector. The higher the ratio of particle scattered light to the background scattered light, the better signal-to-noise ratio. U.S. Pat. Nos. 4,798,465 and 4,893,928 to Knollenberg disclose a detection device to determine particle size from particle effected light scattering in a sensing region illuminated by a laser beam and receiving the particles in a medium, such as air. A linear array of detectors is positioned so that each detector monitors a different portion of the sensing region and provides an electrical output signal that indicates that a particle presence was sensed in the portion that was monitored. The output signals from the detectors are parallel processed and are combined at noise cancellation units with the output signals from other detectors monitoring non-adjacent portions of the sensing region.
In U.S. Pat. No. 4,984,889, Sommer discloses a system to measure particle size using coincidence detection. Light scattered in two different directions from a laser beam by particles are detected by photo-detectors and a coincidence circuit detects simultaneous pulses generated by both photodetectors to discriminate against noise and to disable the integration function in baseline control circuits for preamplifiers amplifying the output signals from the photodetectors.
Other array patents require the particle image to be fixed in space, which is done by having the particle travel towards the collection system. Other prior art methods are not cost effective. None of the prior art methods take all of the necessary steps to significantly reduce most of the light fluctuation noise while detecting the majority of the view volume.